Not that Simple: Email Delivery in the 21st Century
summary
Study runs active experiments with eleven differently configured target domains and passive-DNS ranking to test real-world SMTP behavior. 622 participants from 436 providers attempt 6,842 deliveries, 4,660 arrive, revealing gaps: 65.4% can resolve via IPv6 but only 44.3% actually deliver over IPv6; 41.5% use DNSSEC-validating resolvers; 31.7% publish TLSA; 89.7% accept invalid TLS certs. Greylisting and IPv6-only + TLS enforcement cut spam roughly in half. Measurement infrastructure is published on GitHub.
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